Energy source and environment are important challenges encountered by human being for sustainable development. On the one hand, fossil energy sources are non-renewable, and it is emergent to develop alternative energy sources. On the other hand, the waste gas and sewage generated from the consumption of fossil energy sources have been resulting in severe impact on the environment, which need to be solved synthetically.
Microalgae is a widely distributed lower plant comprising a great deal of categories, which converts the optical energy into a chemical energy of carbohydrates, such as fat or starch, by effective photosynthesis, and thus is called as a “sun driven activating factory”. The generation of biological energy and chemicals by microalgae is hopeful to achieve the dual purposes of substituting the fossil energy sources and cleaning the waste gas and sewage.
In the nature, there is a complicated ecological relationship between microalgae and bacteria. Some specific microalgae and bacteria may benefit one another, while some others may inhibit one another. A known difficulty of cultivating microalgae is the presence of abundant harmful bacteria in water and air, which is unfavorable for the growth of microalgae, even resulting in a failed cultivation. When an open system is used to cultivate microalgae, it is impossible to achieve an aseptic state, then it is under high risk of bacteria contamination. A closed cultivation system with rigorous sterilization can achieve an aseptic state, while for a large scale of microalgae cultivation, it is too expensive.
NOx in an industrial waste gas is one of the significant air pollutants. NOx not only creates photochemical fog and acid rain, but also results in severe greenhouse effect. NOx is also one of the principle inducements of atmospheric haze. The denitration of an industrial waste gas is thus more and more regarded. The processes of denitrating an industrial waste gas can be classified into a dry process and a wet process. Selective Catalytic Reduction (SCR) and Selective non-Catalytic Reduction (SNCR) are conventional dry processes, which both involve high costs of investment and operation, where NOx is reduced to low valuable nitrogen gas without resourcing NOx. The wet process absorbs NOx in a waste gas and immobilizes it in an absorption solution. Such a process has low costs of investment and operation, whilst two problems need to be solved. Firstly, NOx in the industrial waste gas is mainly in the form of NO (generally 90% or more), which is little soluble in water, such that a corresponding means is needed to solve the problem involving the solubility of NO. Secondly, nitrous acid or nitrite is generally unavoidable during the absorption, which is hypertoxicity, such that a corresponding means is needed to the problem involving the separation, re-use or disposal thereof.
On the other hand, nitrogen is one of the nutritive elements consumed most rapidly and most readily lacking during the growth of microalgae. The consumption of a great amount of nitrogenous fertilizer is costly for microalgae cultivation. Therefore, it is desirable to combine the cultivation of microalgae and the denitration of an industrial waste gas, which on one hand can use NOx to provide nitrogenous fertilizer to the microalgae growth, so as to decrease the cost of microalgae cultivation; while on the other hand can purify the waste gas to reduce the discharge of NOx, benefiting the environment. There are some published documents disclosing processes of feeding an industrial waste gas into a cultivating device of microalgae for denitration; however, these processes involve some insoluble problems: (1) the denitration of an industrial waste gas using microalgae must solve the problems restricting the commercialization thereof, such as the illumination and mild climate conditions for cultivating microalgae, while the weather change necessarily resulting in varied efficiencies of denitration, such that the direct feeding of an industrial waste gas is difficult to match the emission operation of the industrial waste gas with the cultivation operation of microalgae, where the two operations interact therebetween to be insufficient to satisfy the requirement of reducing the emission from an industrial production; (2) nitrogen oxide (NO) is the main component of NOx, while NO is little soluble in water, such that the direct feeding of industrial waste gas cannot solve the problem of the great amount of water-insoluble NO in NOx to be little absorbed.
Abundant of NOx is produced from chemical industry. If microalgae is desired to immobilize NOx in the industrial waste gas, the immobilizing rate of NOx by microalgae should match the emitting rate of NOx from the industrial discharge, and the floor space occupied by the microalgae culturing device should be minimized. Generally, the biomass productivity of photoautotrophic microalgae is photoautotrophic cultivation less than 30 g·m−2·d−1, which is reduced to less than 10 g·m−2·d−1 for an outdoor large-scale cultivation. With such a biomass productivity, the plant for denitration of an industrial waste gas will occupy a large area. Thus it is necessary to increase the biomass productivity of microalgae. A heterotrophic or mixotrophic cultivation by adding an organic carbon source is a feasible method of accelerating the growth of microalgae; however, after adding the organic carbon source, the microalgae suspension is quite readily to be polluted by harmful bacteria, resulting in rapid growth of the bacterial significantly faster than that of the microalgae, which even causes a failed microalgae cultivation.
A scaled microalgae cultivation needs abundant water. If water is not recycled, the cultivation is costly. Most of the known categories of microalgae cannot adapt to a high concentration ammonium solution, e.g., ammonium sulphate which is generally used as an inhibitor of microalgae. Meanwhile, when a nitrate is used to provide a nitrogen source to the microalgae, it is difficult to recycle water used in the cultivation, because metal ions accumulates in the cultivating water, resulting in an increased salinity, while a high salinity generally inhibits the growth of microalgae.